In many applications, it is desirable to control the flow of a fluid precisely and to have the ability to change flow control parameters rapidly. Such control may be desired either to maintain a given flow rate in the face of perturbations such as changes in the fluid's flow characteristics or supply pressure or, to effect similarly rapid changes in the flow rate such as may be required to account for changes in the relative speed between the dispenser and a workpiece onto which the fluid is being dispensed.
When dispensing viscous fluids such as certain lubricants, adhesives, sealants and the like, it is often necessary to apply the material to the surface of a workpiece in a bead containing a desired amount of material per unit length In high production processes or where the bead of material must be positioned with accuracy, robot arms are often used to apply the material by rapidly guiding a dispensing nozzle in a programmed pattern over the surface of the workpiece. Depending on the application, the fluid being dispensed may either be projected some distance from the nozzle in a high velocity stream or extruded from the nozzle at lower velocity with the nozzle located closer to the workpiece. In either case, the amount of material applied per unit of lineal distance along the bead will vary according to both the flow rate of material discharged from the dispensing nozzle and the speed of the nozzle with respect to the workpiece.
In the automotive industry, such a process is used to apply a uniform bead of sealant around the periphery of the inside surface of automobile doors before joining the inside panel to the door. Along long, straight portions of the pattern, a robot arm can move the nozzle quickly. However, where the desired bead pattern changes direction abruptly, such as around the corners of a door panel, the robot arm must be slowed down to achieve a required bead positioning accuracy. It can be appreciated that if the flow rate of the dispensed fluid material is held fixed, the amount of material in the applied bead will increase as the robot arm is decelerated to negotiate changes in direction and will decrease as the robot arm is accelerated. Likewise, changes in the fluid supply pressure or changes in the viscosity of the fluid material will tend to disrupt control over the size of the bead.
An apparatus and method which effectively addresses these difficulties is fully described in co-pending U.S. Patent Application S/N 06/924,940, assigned to the same assignee as the present invention and expressly incorporated herein by reference. That application discloses, inter alia, a fluid dispensing method and apparatus wherein a servo actuator drives a variable fluid metering valve located in close proximity to a fluid discharge nozzle. The valve is a needle valve comprising a valve seat and a stem moveable relative the seat to vary the flow through the seat. A pressure sensor at the nozzle generates a signal correlated to the instantaneous flow rate of the dispensed fluid. Control over flow is achieved by connecting the dispenser in a closed-loop system in which the actuator is driven by a control current derived in accordance with the difference between the flow rate signal and a driving signal representing a desired flow rate In robotic applications, the driving signal is preferably related to a toolspeed signal emanating from the controller of the robot carrying the dispenser so that the control current will vary as required to maintain a uniform bead of fluid material even during relatively rapid changes in the relative speed between the dispenser and the workpiece onto which material is dispensed.
The stem-and-seat needle valve arrangement used in the device disclosed in the above-referenced patent application falls within a class of valves which may be described as variable-area flow restrictors. Other valves in this class include gate valves and shutter valves. Valves in this class modulate flow by varying an area through which flow may take place such as the area between a valve stem and its seat.
It is an inherent characteristic of such valves that, as the valve closes, its sensitivity increases. That is, for a given percentage of actuation, the corresponding percentage change in the flow through the valve will be greater when the valve is nearly closed than when it is more fully open. This is due to geometrical factors in that a given amount of actuation results in a greater change in flow area when the valve operates near the "closed" end of its range than when the valve operates more toward the "open" end of its range.
When a variable-area flow restrictor type valve is employed as a metering valve in a closed-loop dispensing system which must operate accurately and rapidly over a range of flow rates, including flow rates where the metering valve is nearly closed, the above characteristic limits system performance. Since the valve is quite sensitive when nearly closed, system stability is decreased when the metering valve is so positioned. This limits the maximum gain at which the system can operate which, in turn, limits system response time so that when the valve is operating at the more open end of its range, response time is slower than required to maintain stability when the valve is operating at the more closed end of its range.